Adam Walton has learned the hard way you can't afford to miss your big opportunities in top-flight tennis as Australia's rising men's hope got a tough lesson at the Miami Open.

Walton, who had battled superbly to get through the qualifiers at the prestigious Masters 1000 event in Florida, was handed the biggest match of his burgeoning career when he was paired with former teen prodigy Felix Auger-Aliassime in Thursday's first round.

The 24-year-old from Brisbane, who's enjoying a breakthrough year to get to No.150 in the world, gave the one-time world No.6 a terrific battle in the opening set, and, when 5-4 up, even worked himself into the position of earning three set points to draw first blood.

But after he missed one golden opportunity to clinch the set, spraying what should have been a straightforward forehand winner horribly wide into the tramlines, it marked the beginning of the end for Walton's sprightly challenge.

Canadian Auger-Aliassime, who's had some struggles with injuries and form and has dipped to No.36 in the world, went on to hold serve and immediately break the deflated Queenslander's delivery in the next game before serving out to take the set 7-5.

Another solitary break in the second set after Walton had battled hard to get to 4-4 was then enough for two-time grand slam quarter-finalist Auger-Aliassime, with his superior firepower and experience, to seal the 7-5 6-4 win after exactly two hours.

The Canadian victor was impressed with the Aussie, who'd been playing just his third tour-level match.

"I wasn't doing anything too bad in the first set, I thought he was just playing really well, serving to his targets, playing high percentage - he was making me work," Auger-Aliassime shrugged.

Despite the loss, his run at Miami Gardens will see Walton enjoy a career-best ranking next week - provisionally, he's moved to 136 - in a season when he's already won one Challenger tournament in Burnie, been in the final of two others and earned his first wins over top-100 opponents.

In contrast, Alexei Popyrin didn't encounter too many hurdles in reaching the second round.

The Australian romped to a straight-forward 6-2 6-2 win over Facundo Diaz Acosta to set up a meeting with No.26 seed Jiri Lehecka in the next round.

Popyrin held serve throughout and broke the Argentine four times in a dominant performance, winning 66 points to Diaz Acosta's 45.

It was much closer though for Aleksandar Vukic, who bowed out in a

tight 7-6 (7-2) 6-3 loss to Roberto Carballes Baena.

Vukic sent down six aces but was broken once in the second set - all the Spaniard needed to secure the victory.

The Sydneysider forced three break-point opportunities of his own but couldn't convert any.

Friday also saw the return of the popular Japanese Kei Nishikori, a former Miami finalist who's battling to reignite his injury-beset career at 34 after being out of action since last July, but there was to be no fairytale comeback as he was beaten 6-3 6-4 by Austria's Sebastian Ofner.